Octave nomenclature

Middle C (the fourth C key from left on a standard 88-key piano keyboard) is designated C4 in scientific pitch notation, the most commonly recognized in auditory science[], while both C4 and the Helmholtz designation c' are used in musical studies. Other note-octave systems, including those used by some makers of digital music keyboards, may refer to Middle C differently. In MIDI, Middle C is note number 60.

While the expression Middle C is generally clear across instruments and clefs, some musicians naturally use the term to refer to the C note in the middle of their specific instrument's range. C4 may be called Low C by someone playing a Western concert flute, which has a higher and narrower playing range than the piano, while C5 (523.251 Hz) would be Middle C. This technically inaccurate practice has led some pedagogues to encourage standardizing on C4 as the definitive Middle C in instructional materials across all instruments.[1]

In vocal music, the term Soprano C, sometimes called High C[2] or Top C[by whom?], is the C two octaves above Middle C. It is so named because it is considered the defining note of the soprano voice type. It is C6 in scientific pitch notation (1046.502 Hz) and c''' in Helmholtz notation. The term Tenor C is sometimes used[by whom?] in vocal music to refer to C5, as it is the highest required note in the standard tenor repertoire. The term Low C is sometimes used in vocal music to refer to C2 because this is considered the divide between true basses and bass-baritones: a basso can sing this note easily while other male voices, including bass-baritones, cannot.

Tenor C is an organ builder's term for small C or C3 (130.813 Hz), the note one octave below Middle C. In stoplists it usually means that a rank is not full compass, omitting the bottom octave.[3]